1. Field of the Invention
The present invention relates to a patch antenna in which a feed pin for feeding is soldered to a patch electrode on a dielectric substrate and, in particular, to a solder joint structure of the feed pin.
2. Description of the Related Art
In recent years, in accordance with the widespread use of wireless communication systems, such as Global Positioning Systems (GPS) and Electronic Toll Collection systems (ETC), a demand for patch antennas, which are planar and ultra-compact, has been growing. Such a patch antenna has a patch electrode made of copper or silver on a ceramic dielectric substrate. A given high-frequency signal is fed to the patch electrode to resonate so that the patch antenna transmits and receives a signal wave in the resonance frequency band. In general, a ground conductor plate is attached to the bottom surface of the dielectric substrate, which is disposed on a circuit board having a low noise amplifier (LNA). Feeding to the patch electrode is carried out via a feed pin, for example, shown in FIG. 5.
Referring to FIG. 5, a through-hole 2a is formed at the position corresponding to a feeding point of a patch electrode 3 in the dielectric substrate 2 of the patch antenna 1. A feed pin 4 is inserted into the through-hole 2a and a head 4a of the feed pin 4 is coupled to the patch electrode 3 with a solder 5 electrically and mechanically. The feed pin 4 is made of metal having high electrical conductivity, such as brass. The bottom end of the feed pin 4 (not shown) extends through a circuit board (not shown), which mounts the patch antenna, and is soldered to a low noise amplifier on the bottom surface of the circuit board. The head 4a of the feed pin 4 is a flat plate. The head 4a is disposed on the patch electrode 3 and soldered thereto, as is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 9-8537 (page 3 and FIG. 1 of the cited document).
Referring to FIG. 5 showing the known patch antenna 1, there is a significant difference in the coefficients of thermal expansion between the dielectric substrate 2 made of ceramic and the feed pin 4 made of brass. Consequently, a significant change of environmental temperature causes deformation of the dielectric substrate 2 due to thermal contraction or thermal expansion of the head 4a of the feed pin 4. Reaction force to recover from the deformation acts on the solder 5 as shearing stress. If the patch antenna 1 stays for a long time in an environment where temperature variation is significant and thus thermal contraction or thermal expansion of the head 4a of the feed pin 4 frequently occurs, a crack in the solder 5 is produced to push up the head 4a from the patch electrode 3, causing defective electrical continuity between the feed pin 4 and the patch electrode 3. Such a defective soldered joint of the feed pin 4 and the patch electrode 3 due to the temperature variation particularly occurs in a low temperature environment.